netdev - Re: [RFC PATCH net-next v6 00/15] Device Memory TCP

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Message-ID: <de633244-6d6e-f568-dc09-49b56abd0423@huawei.com>
Date: Wed, 6 Mar 2024 20:37:35 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Mina Almasry <almasrymina@...gle.com>
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Subject: Re: [RFC PATCH net-next v6 00/15] Device Memory TCP

On 2024/3/6 3:38, Mina Almasry wrote:
> On Tue, Mar 5, 2024 at 4:54 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>>
>> On 2024/3/5 10:01, Mina Almasry wrote:
>>
>> ...
>>
>>>
>>> Perf - page-pool benchmark:
>>> ---------------------------
>>>
>>> bench_page_pool_simple.ko tests with and without these changes:
>>> https://pastebin.com/raw/ncHDwAbn
>>>
>>> AFAIK the number that really matters in the perf tests is the
>>> 'tasklet_page_pool01_fast_path Per elem'. This one measures at about 8
>>> cycles without the changes but there is some 1 cycle noise in some
>>> results.
>>>
>>> With the patches this regresses to 9 cycles with the changes but there
>>> is 1 cycle noise occasionally running this test repeatedly.
>>>
>>> Lastly I tried disable the static_branch_unlikely() in
>>> netmem_is_net_iov() check. To my surprise disabling the
>>> static_branch_unlikely() check reduces the fast path back to 8 cycles,
>>> but the 1 cycle noise remains.
>>>
>>
>> The last sentence seems to be suggesting the above 1 ns regresses is caused
>> by the static_branch_unlikely() checking?
> 
> Note it's not a 1ns regression, it's looks like maybe a 1 cycle
> regression (slightly less than 1ns if I'm reading the output of the
> test correctly):
> 
> # clean net-next
> time_bench: Type:tasklet_page_pool01_fast_path Per elem: 8 cycles(tsc)
> 2.993 ns (step:0)
> 
> # with patches
> time_bench: Type:tasklet_page_pool01_fast_path Per elem: 9 cycles(tsc)
> 3.679 ns (step:0)
> 
> # with patches and with diff that disables static branching:

> time_bench: Type:tasklet_page_pool01_fast_path Per elem: 8 cycles(tsc)
> 3.248 ns (step:0)
> 
> I do see noise in the test results between run and run, and any
> regression (if any) is slightly obfuscated by the noise, so it's a bit
> hard to make confident statements. So far it looks like a ~0.25ns
> regression without static branch and about ~0.65ns with static branch.
> 
> Honestly when I saw all 3 results were within some noise I did not
> investigate more, but if this looks concerning to you I can dig
> further. I likely need to gather a few test runs to filter out the
> noise and maybe investigate the assembly my compiler is generating to
> maybe narrow down what changes there.

Yes, that is confusing enough that need more investigation.

>